1. Field of the Invention
The present invention relates to a memory device and a method for manufacturing the same, and more particularly, to a memory device which is a single electron device and a method for manufacturing the same.
2. Description of Related Art
A semiconductor memory device includes two fundamental components: a transistor used as a switch for obtaining current paths, when writing or reading information onto or from a capacitor, and a capacitor for keeping the stored electric charges.
It is necessary for a transistor to have a high transconductance (gm) characteristic so that a great deal of current flows in the transistor. Therefore, recent memory devices include Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) adapted as switching devices, since they have a high transconductance. The MOSFET is a transistor including two fundamental components, i.e., a gate electrode formed of doped polycrystalline silicon, and a source electrode and a drain electrode formed of doped crystalline silicon.
Recently, research has been carried out to try and reduce the size of a device to embody a highly integrated memory device. The smaller the size of a device, the more devices can be integrated in a unit volume, and the less time is required for transmitting signals between the devices. Accordingly, the miniaturization of devices is advantageous for high speed handling of a great amount of information.
However, the existing MOSFET generates a large amount of heat. If a lot of devices adapting the MOSFET are integrated in a small area, the devices may melt down or cause malfunctions.
To overcome the above problems, one of the next generational devices under development is a single electron device SED. A single electron device refers to a device using a Coulomb Blockade effect, i.e., a phenomenon in which an electron is blocked from tunneling into a dot by the classical Coulomb repulsive force of another electron which already exists inside the dot.
In general, the current-voltage characteristic of a tunnel junction having a size of less than 100 nm and a relatively high resistance does not follow ohm's law and a current generated by transmitting electrons under low voltage is reduced to almost 0. This phenomenon occurs when a transmitted electron interacts with an electric field charged with the electron by the electrostatic force. This phenomenon was published in a theory based on quantum mechanics by Likharev et al. in 1986 and observed in an artificial infinitesimal junction in 1987 for the first time.
Theoretically, the single electron device can operate with a single electron under a certain temperature. Here, the certain temperature refers to a temperature of which a thermal energy KBT lower than the electrostatic energy e2/C which is required with an electron in a junction inside a device; in other words, a temperature which meets the following equation.
                    T        ⁢                  〈                      〈                                          ⅇ                2                                                              K                  B                                ⨯                C                                                                        [                  mathematic          ⁢                                          ⁢          formula          ⁢                                          ⁢          1                ]            
Here, KB(=1.38×10−23 J/K) is the Boltzmann's constant. Capacitance C increases in proportion to the rise in temperature of a junction. Therefore it is possible to observe the Coulomb Blockade effect at any temperature, if only the size of the junction is reduced. It is notable that the size of the junction should be reduced to less than 10 nm×10 nm and the size of a quantum dot for storing an electrode should also be reduced to less than 10 nm. However, it is difficult to obtain a junction or a quantum dot of this size.